Electro-optic modulator utilizing copper-tungsten electrodes for improved thermal stability

ABSTRACT

A high-power electro-optic modulator (EOM) is formed to use specialized electrodes of a material selected to have a CTE that matches the CTE of the modulator&#39;s crystal. Providing CTE matching reduces the presence of stress-induced birefringence, which is known to cause unwanted modulation of the propagating optical signal. The specialized electrodes are preferably formed of a CuW metal matrix composite having a W/Cu ratio selected to create the matching CTE value. Advantageously, the CuW-based electrodes also exhibit a thermal conductivity about an order of magnitude greater than conventional electrode material (brass, Kovar) and thus provide additional thermal stability to the EOM&#39;s performance.

TECHNICAL FIELD

The present invention relates to high power electro-optic modulatorsand, more particularly, to the utilization of electrodes specificallyformed to exhibit a coefficient of thermal expansion (CTE) to match theCTE of the modulator's crystal material.

BACKGROUND OF THE INVENTION

An electro-optic modulator (EOM) is a device in which an electricalsignal can be imposed on an optical carrier. This electrical signal maybe imposed on the phase, frequency, amplitude, or polarization of theoptical carrier, depending on the orientation of the includedelectro-optic active material (crystal) and the system in which in themodulator is used. The imposed electrical signal functions to modify therefractive index of the electro-optic active material, which in turnmodifies (i.e., “modulates) the optical carrier passing through thecrystal. A polarization modulator may be configured as avoltage-controlled shutter capable of switching a laser source at ratesof kHz-MHz with rise/fall times on the order of nanoseconds. EOMs usedin such a configuration are susceptible to degradation of performancedue to thermal effects, including undesired leaking of light via stressbirefringence, pointing shift, and thermal lensing.

One approach to address the thermal issues related to high power EOMs isto maintain the modulator package itself at a stable temperature, suchas by the utilization of water cooling. Water cooling, however,introduces additional complexities to the package design and obviouslyrequires extra facilities at the modulator installation site.

SUMMARY OF THE INVENTION

The needs remaining in the prior art are addressed by the presentinvention, which relates to the utilization of specialized electrodesparticularly formed to exhibit a coefficient of thermal expansion (CTE)that essentially matches the CTE of the modulator's crystal material.

In accordance with the principles of the present invention, athermally-stable EOM is provided by utilizing specializedcopper-tungsten (CuW) metal matrix composite electrodes formed to have aspecific W/Cu ratio such that the CTE of the electrodes substantiallymatches the CTE of the active crystal material. This is in contrast toprior art configurations that utilize electrodes formed of either brassor a nickel-cobalt ferrous alloy (sold under the tradename of “Kovar”),neither material particularly well-suited in terms of matching CTE.

It is an aspect of the present invention that utilizing electrodeshaving a CTE that is matched to the active crystal materialsignificantly reduces (indeed, substantially eliminates) unwantedstress-related birefringence in the crystal as the modulator temperaturefluctuates, since any expansion/contraction of the electrodes as afunction of temperature will be essentially the same as thoseexperienced by the crystal. Minimizing this stress-based birefringenceminimizes leakage of light across the crystal, maintaining consistentswitching behavior based solely upon the voltage-induced birefringencerelated to the electrical modulating signal.

Moreover, CuW metal matrix composite electrodes advantageously exhibit ahigher thermal conductivity than conventional electrodes formed ofmaterials such as Kovar or brass. As a result, the use of CuW metalmatrix composite electrodes in accordance with the principles of thepresent invention allows for the heat generated within the crystal to bequickly transported away and thus reduce temperature-induced changes incrystal behavior (e.g., pointing shift, thermal lensing).

The combination of reduced thermal stress and improved thermalconductivity yields an electro-optic modulator that is able to operationover a wide temperature range with consistent results.

The CuW metal matrix composite electrodes may be mechanically clamped tothe crystal, or bonded to the crystal using specific thin films thatfurther improve thermal conductivity. The bonding may take the form of acold weld bond or a soldered bond. Indeed, one possible configurationmay utilize one electrode that is mechanically clamped to the crystaland one electrode that is bonded to the crystal.

The active crystal material, generally speaking, is defined as a singlecrystal material whose crystal structure lack inversion symmetry, andexhibits electro-optic effects. Examples of such single crystalmaterials useful in a modulator of the present invention include, butare not limited to, the following: CdTe, CdZnTe, ZnTe, and GaAs. Inaccordance with the principles of the present invention, the specificformulation of the W/Cu ratio in the metal matrix composite is createdso that the CTE of the electrodes essentially matches that of materialselected for use as the crystal.

An exemplary embodiment of the present invention may take the form of anelectro-optic modulator comprising a single crystal element thatexhibits changes in internal birefringence in response to an appliedvoltage (the single crystal element having opposing major surfaces andexhibiting a material-specific coefficient of thermal expansion (CTE))The electro-optic modulator also includes a pair of electrodes formed ofa CuW metal matrix composite and coupled to the opposing major surfacesof the single crystal element. In accordance with the principles of thepresent invention, the electrodes are formed to have a W/Cu ratio suchthat the CTE of the CuW metal matrix composite electrodes essentiallymatches the CTE of the single crystal element.

Other and further aspects and advantages of the present invention willbecome apparent during the course of the following discussion and byreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings,

FIG. 1 is a simplified block diagram of an exemplary electro-opticmodulator formed in accordance with the principles of the presentinvention;

FIG. 2 is a graph showing CTE as a function of % W in a CuW metal matrixcomposite material;

FIG. 3 shows one embodiment of a combination of an exemplary crystal andpair of specialized electrodes, with the electrodes physically bonded toopposing major surfaces of the crystal; and

FIG. 4 shows another embodiment of this combination, where thespecialized electrodes are mechanically attached to (i.e., “clamped” to)the opposing major surfaces of the crystal.

DETAILED DESCRIPTION

Reference throughout this specification to an “example” or an“embodiment” means that a particular feature, structure, orcharacteristic described in connection with the example is included inat least one embodiment of the invention. Thus appearances of the terms“example” or “embodiment” in various places throughout the specificationdo not necessarily refer to the same embodiment. Furthermore, as usedherein, the terms “about”, “substantially” and “essentially” means thatthe recited characteristic (e.g., “CTE”) need not be achieved exactly,but that deviations or variations, including for example, tolerances,measurement error, measurement accuracy limitations and the like asknown to those skilled in the art, may occur in amounts that do notpreclude the effect that the recited characteristic was intended toprovide.

FIG. 1 illustrates an exemplary electro-optic modulator 10 formed inaccordance with the present invention. The operation of electro-opticmodulator 10 is based upon the ability to “modulate” the birefringenceof an active single crystal component 12 by the application of anelectric field potential ε across the crystal. A pair of electrodes 14,16 is disposed to contact opposing major surfaces 12A, 12B of crystalcomponent 12. As also shown, electrodes 14, 16 are coupled to anexternal electrical signal source 18 (i.e., a data signal for modulatingthe birefringence so as to impress the data signal on an opticalcarrier). When the electric field is present, an internal birefringenceis created and causes a rotation of the polarization direction ofcrystal component 12. As a result of this polarization rotation, anoptical signal propagating through crystal component 12 undergoes apolarization transformation as it progresses from an input endface 12 ato an output endface 12 b of crystal component 12.

In the particular configuration of FIG. 1 , crystal component 12 ispositioned between a first polarizing element 20 and a second polarizingelement 22, where the polarization state of second element 22 is rotated90° with respect to first element 20 (in some cases, second polarizingelement 22 is referred to as an “analyzer”).

An input optical carrier beam initially passes through first polarizingelement 20 so that a beam of a “known” polarization state is created.The polarized beam then passes through crystal 12 and is rotated througha predetermined polarization angle proportional to the electric field εcreated by the presence of a voltage across electrodes 14, 16. Theamplitude of the modulated optical signal exiting second polarizingelement 22 will thus be a function of the electric field applied tocrystal 12 (i.e., indicative of the orientation between the polarizedinput signal and the electric field-induced polarization state of thecrystal).

The arrangement of FIG. 1 thus functions as a voltage-controlledshutter, capable of switching a laser at rates of kHz-MHz, withrise/fall times on the order of nanoseconds. However, the generation ofheat within crystal 12 is known to be problematic, allowing foradditional, unwanted modulation of the propagating optical signal tooccur. Indeed, one source of temperature increase within crystal 12 isthe propagation of the optical carrier beam itself, which necessarilyresults in the absorption of optical energy within the crystallinematerial, thus elevating its temperature.

One undesirable effect of the increase in crystal temperature is“thermal lensing”, which is a known effect where those regions of thecrystal that experience a larger change in temperature exhibit a greaterchange in refractive index. The presence of a temperature gradientacross the crystal results in forming a refractive index gradient thatadversely impacts the optical beam propagating through the crystal.Moreover, the presence of elevated temperatures is likely to introduceunwanted stress-induced birefringence within the crystal, due to a CTEmismatch between the crystal and conventional materials used to form theelectrodes.

Therefore, in accordance with the principles of the present invention,electrodes 14, 16 are formed of a specialized material having a CTE thatis particularly designed to match the CTE of crystal component 12. Thespecialized material preferably comprises a CuW metal matrix structure,with the ratio of W to Cu controlled to provide CTE matching (“matching”in this case defined as substantially the same value, within limits asmentioned above).

By matching CTE, the possibility of temperature-related, unwantedstress-induced birefringence within crystal 12 (related to strain-stressat the interface between crystal 12 and electrodes 14, 16) issignificantly reduced, if not substantially eliminated. As a result, thechange in birefringence experienced by crystal 12 will be controlledsolely by the application (and the switching) of the voltage acrosselectrodes 14, 16, stabilizing the modulator performance in high power,high temperature operating conditions.

Moreover, it has been found that CuW exhibits a thermal conductivity onthe order of magnitude greater than that of conventional electrodematerials (e.g., brass, Kovar). This relatively high thermalconductivity of CuW (greater than 100 W/mK, as compared to the 17 W/mKthermal conductivity of Kovar) improves the cooling of the modulatorcrystal, reducing pointing shift and thermal lensing. Indeed, the use ofspecialized CuW metal matrix composite electrodes in accordance with theprinciples of the present invention allows high-power EOMs to operate ina less-exotic environment than water-cooled arrangements. It is to beunderstood, however, that the inventive CTE-matched electrode/crystalconfiguration of the present invention may still be used in awater-cooled modulator installation.

In embodiments where the single crystal material selected for use isCdTe (having a CTE of 5.9*10⁻⁶/K), CuW metal matrix composite electrodesformed to have a W/Cu ratio of 93/7 will exhibit essentially this sameCTE. In high-power EOM embodiments using GaAs crystals (having a CTE of5.6*10⁻⁶/K), using a W/Cu ratio of 95/5 yields a CuW metal matrixcomposite electrode with this same CTE value. FIG. 2 is graphillustrating the CTE of CuW as a function of the percentage of W in themetal matrix structure, which may therefore be used in determine aspecific W/Cu ratio best suited for a particular crystal material. SinceCu and W are not mutually soluble, the electrode material is composed offinely dispersed copper and tungsten phases. In one form, a selectedvolume of molten Cu is used to infiltrate the W matrix to yield a finalproduct have the desired, specialized W/Cu ratio required for CTEmatching with the modulator's crystal component.

As mentioned above, there are a variety of different configurations thatmay be used to couple the specialized CuW metal matrix electrodes to theelectro-optic crystal material in order to form a thermally-stable EOMin accordance with the principles of the present invention. FIG. 3illustrates one exemplary arrangement where electrodes 14, 16 aredirectly bonded to crystal 12 by a thin layer 30 of an electrically- andthermally-conductive adhesive. The adhesive is applied to the opposingmajor surfaces 12A, 12B of crystal 12 to form thin layers 30, andelectrodes 14, 16 are then brought into contact with the adhesive. Inother embodiments, appropriate metal films may be applied to both theexposed major surfaces of crystal 12 and the mating surfaces ofelectrodes 14, 16. The metal film is used to create either a cold weldbond or a soldered bond (depending on the composition of the metalfilm).

FIG. 4 illustrates another possible arrangement for coupling electrodes14, 16 to crystal 12. Here, crystal 12 is mechanically clamped betweenelectrodes 14, 16. A pair of clamps 40, 42 is shown in FIG. 4 , wherethe directional arrows depict the mechanical force (F) used to holdclamps 40, 42 in place against electrodes 14 and 16, respectively. Theclamping also creates the necessary electrical signal path (i.e.,creating the electric field across crystal 12). While not explicitlyshown, it is also possible to utilize an arrangement where one electrodeis physically attached (bonded) to crystal 12 (as depicted in FIG. 3 )and the other electrode is held in place mechanically against thesurface of crystal 12 (as depicted in FIG. 4 ).

It will be clear to those skilled in the art that various modificationsand adaptations can be made to the present invention without departingfrom the spirit and scope thereof. Accordingly, the inventive scope isnot to be limited by the specific embodiments described herein. Rather,scope of the invention is to be defined by the following claims andtheir equivalents.

What is claimed is:
 1. An electro-optic modulator comprising a singlecrystal element that exhibits changes in internal birefringence inresponse to an applied voltage, the single crystal element havingopposing major surfaces and exhibiting a material-specific coefficientof thermal expansion (CTE); and electrodes formed of a CuW metal matrixcomposite and coupled to the opposing major surfaces of the singlecrystal element, a W/Cu ratio of the CuW metal matrix composite selectedto create electrodes having a CTE that substantially matches the CTE ofthe single crystal element.
 2. The electro-optic modulator as defined inclaim 1 wherein the electrodes are mechanically clamped to the opposingmajor surfaces of the single crystal element.
 3. The electro-opticmodulator as defined in claim 1 wherein the electrodes are bonded to theopposing major surfaces of the single crystal element.
 4. Theelectro-optic modulator as defined in claim 3 wherein a thin adhesivefilm is disposed between the opposing major surfaces of the singlecrystal element and the electrodes.
 5. The electro-optic modulator asdefined in claim 3 wherein a thin metal film is disposed between theopposing major surfaces of single crystal element and the electrodes. 6.The electro-optic modulator as defined in claim 1 wherein single crystalelement is selected from the group consisting of: CdTe, CdZnTe, ZnTe,and GaAs.
 7. The electro-optic modulator as defined in claim 6 whereinthe single crystal element comprises CdTe, having a material-based CTEof 5.9*10⁻⁶/K, and the W/Cu ratio is selected to be 93/7 to formCTE-matched electrodes of CuW metal matrix composite material.
 8. Theelectro-optic modulator as defined in claim 6 wherein the single crystalelement comprises GaAs, having a material-based CTE of 5.6*10⁻⁶/K, andthe W/Cu ratio is selected to be 95/5 to form CTE-matched electrodes ofCuW metal matrix composite material.